/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.dx.io;

import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.zip.Adler32;

/**
 * Generates and stores the checksum and signature of a dex file.
 */
public final class DexHasher {

	private static final int CHECKSUM_OFFSET = 8;
	private static final int CHECKSUM_SIZE = 4;
	private static final int SIGNATURE_OFFSET = CHECKSUM_OFFSET + CHECKSUM_SIZE;
	private static final int SIGNATURE_SIZE = 20;

	/**
	 * Returns the signature of all but the first 32 bytes of {@code dex}. The
	 * first 32 bytes of dex files are not specified to be included in the
	 * signature.
	 */
	public byte[] computeSignature(DexBuffer dex) throws IOException {
		MessageDigest digest;
		try {
			digest = MessageDigest.getInstance("SHA-1");
		} catch (NoSuchAlgorithmException e) {
			throw new AssertionError();
		}
		int offset = SIGNATURE_OFFSET + SIGNATURE_SIZE;

		byte[] bytes = dex.getBytes();
		digest.update(bytes, offset, bytes.length - offset);
		return digest.digest();
	}

	/**
	 * Returns the checksum of all but the first 12 bytes of {@code dex}.
	 */
	public int computeChecksum(DexBuffer dex) throws IOException {
		Adler32 adler32 = new Adler32();
		int offset = CHECKSUM_OFFSET + CHECKSUM_SIZE;

		byte[] bytes = dex.getBytes();
		adler32.update(bytes, offset, bytes.length - offset);
		return (int) adler32.getValue();
	}

	/**
	 * Generates the signature and checksum of the dex file {@code out} and
	 * writes them to the file.
	 */
	public void writeHashes(DexBuffer dex) throws IOException {
		byte[] signature = computeSignature(dex);
		dex.open(SIGNATURE_OFFSET).write(signature);

		int checksum = computeChecksum(dex);
		dex.open(CHECKSUM_OFFSET).writeInt(checksum);
	}
}
